1. Field of the Invention
The present invention relates to the domain of semiconductor components, especially solar cells, and relates to a crystalline solar cell having an n- or p-doped semiconductor substrate and a passivated backside.
2. Description of the Related Art
Silicon solar cells having a “passivated backside” have improved optical aluminizing and a greatly improved passivation of the back surface compared to the aluminum back surface field (BSF) that has been produced in a standard manner up to now, cf. A Götzbarger et al., “Sonnenenergie: Photovoltaik” (Solar Energy: Photovoltaics), B. G. Teubner Stuttgart, 1997. The cell concept produced thereby is called “Passivated Emitter and Rear Cell” (PERC). In this context, the dielectric passivation adapted to the backside doping is opened locally at many small points, so that the metal layer deposited on the passivating layer is not able to contact the semiconductor, but only at a small surface portion of the backside, in order to minimize the strong recombination of the electron hole pairs at metallized surfaces.
The metallizing of the backside is made up in most cases of aluminum, and is deposited over a large surface on the entire backside, as a rule, using vacuum vapor deposition technology or sputtering.
Documents relating to such solar cells as well as methods for their production, and are connected to backside patterning steps and/or the backside driving in of doping substances, are the likes of published German patent document DE 195 25 720 C2, published German patent application document DE 10 2007 059 486 A1 or published German patent application document DE 10 2008 013 446 A1, as well as published German patent application document DE 10 2008 033 169 A1 (both of the latter from ErSol Solar Energy AG).
Such documents as published Japanese patent application document JP 2005 027 309 A, DE 10 2008 017 312 A1 or published German patent application document DE 10 2008 020 796 A1 are concerned with the efficient production of reliable connecting patterns, particularly those having soldered connections.
The method “laser fired contacts” (LFC) is known, in which the metallization is fired through on the backside passivation using laser pulses, so that a prior opening of the passivating layer becomes unnecessary, cf. “Laserstrahlverfahren zur Fertigung kristalliner Silizium-Solarzellen” (Laser Beam Method for Producing Crystalline Silicon Solar Cells), Dissertation by Eric Schneiderlöchner, Albert-Ludwigs-Universität Freiburg im Breisgau (2004) or (earlier) published German patent application document DE 199 15 666 A1.
In the case of p-doped wafers, the formation of “local BSF regions” at the contact points in the backside passivation is undertaken simply by alloying the aluminum into the p− or p+ surface. This takes place at temperatures above the Al—Si eutectic temperature of 577° C. For this it is necessary that the backside passivation layer and also the front side emitter (after the sintering of the front side silver paste) survive these temperatures undamaged.
In the case of n-doped wafers, in which the emitter (p-n junction) is produced on the backside using boron or aluminum doping, the formation of the Al—Si eutectic, which, as a rule, would melt a few micrometers in depth, would lead to a breakthrough through the thin p+ emitter into the n-base and there it would lead to a short circuit to the base. For that reason, the metallization has to be tempered at low temperatures (e.g. 400° C., optionally also in forming gas), in order to produce a sufficiently good ohmic contact to the emitter surface, without damaging the cell. This makes impossible the firing of a normal screen printing paste for the subsequent depositing of a silver layer, that is able to be soldered, on the aluminum.
All the PERC technologies, known from the related art, have the following disadvantages:    1.) It is common to all the concepts that the backside is present at the end of the process having a large-surface aluminum layer, which does not have solderable pad areas.    2.) The low ohmicity of the metallization required for the current conduction is only able to be produced via a sufficiently thick aluminum layer, as a rule, 2-4 μm. If the vapor deposition or sputter rate is selected to be high, in order to keep the processing times low, the heat input, and with that, the temperature of the wafer in the process chamber, increase greatly. If the rate is held to be low, more time or a longer passage system having additional deposit or sputtering sources for the aluminum coating have to be provided. In any case, the compromise is connected to higher investment and or processing costs.    3.) One alternative is a thinner aluminum layer which is able to be produced in a sufficiently short period of time at a moderate deposition rate, but then a chemical or galvanic reinforcement of the Al backside metallization is required. This takes place, though, at very moderate temperatures (<90° C.) and thus represents no thermal stress of the almost ready solar cells.    4.) If one found a suitable method for reinforcing aluminum chemically or galvanically, however, the entire backside would be reinforced with silver, which would represent a considerable cost factor.